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Maternal exposure to stressors during lactation has previously been demonstrated to impact various aspects of milk synthesis and to have long-term physiological effects on offspring. Much of the current literature investigating the effects of stress during lactation has used acute stressors, and the studies investigating the effects of chronic stressors largely focus on neurological changes. Further, temporal variation in glucocorticoids across lactation in response to stressors has rarely been assessed. The present work uses a novel male intruder paradigm to model the effects of chronic stress on maternal fecal corticosterone metabolites (FCMs) in Sprague-Dawley rats across lactation. FCM levels were elevated in chronically-stressed mothers relative to the control group. Further, FCMs in the stress group were time-dependent either due to repeated exposure to the stressor or lactation stage. Together, this work demonstrates the efficacy of this established paradigm in increasing circulating glucocorticoids in lactating rats. These results highlight the need for repeated temporal sampling, as glucocorticoid levels in response to a chronic stressor may change across lactation., (Copyright © 2021 Elsevier Inc. All rights reserved.)

The ability to provision offspring with milk is a significant adaptive feature of mammals that allows for considerable maternal regulation of offspring beyond gestation, as milk provides complete nutrition for developing neonates. For mothers, lactation is a period of marked increases in energetic and nutritive demands to support milk synthesis; because of this considerable increase in demand imposed on multiple physiological systems, lactation is particularly susceptible to the effects of chronic stress. Here, we present work that explores the impact of chronic stress during lactation on maternal lactation performance (i.e. milk quality and quantity) and the expression of key milk synthesis genes in mammary tissue using a Sprague–Dawley rat model. We induced chronic stress using a well-established, ethologically relevant novel male intruder paradigm for 10 consecutive days during the postpartum period. We hypothesized that the increased energetic burden of mounting a chronic stress response during lactation would decrease lactation performance. Specifically, we predicted that chronic exposure to this social stressor would decrease either milk quality (i.e. composition of proximate components and energy density) or quantity. We also predicted that changes in proximate composition (i.e. lipid, lactose, and protein concentrations) would be associated with changes in gene expression levels of milk synthesis genes. Our results supported our hypothesis that chronic stress impairs lactation performance. Relative to the controls, chronically stressed rats had lower milk yields. We also found that milk quality was decreased; milk from chronically stressed mothers had lower lipid concentration and lower energy density, though protein and lactose concentrations were not different between treatment groups. Although there was a change in proximate composition, chronic stress did not impact mammary gland expression of key milk synthesis genes. Together, this work demonstrates that exposure to a chronic stressor impacts lactation performance, which in turn has the potential to impact offspring development via maternal effects. [ABSTRACT FROM AUTHOR]